Automatically producing an optical blend mask individually adapted to a projector and its position to a projection surface of the projection system

ABSTRACT

A method for automatically producing an optical blend mask arranged in a beam path (8) of a projector (2) in a projection system with two projectors (2), by determining an actual state of the projection system by calibrating and producing an ideal blending, ascertaining an individual distortion of the projector (2) by using a front or back projection surface (11), introduced into the beam path (8), using patterns (13) from which points are derived, ascertaining the alignment and the position of the front or back projection surface (11) within a blend mask plane, calculating the optical blending for a partial image, for positioning and/or a transformation of the ideal blending, adapting the ideal blending for compensating a soft focus, applying the ascertained, individual distortion of the projector (2) to the blending, ascertaining data of the optical blend mask and outputting data for producing the optical blend mask.

This is an application filed under 35 USC § 371 of PCT/DE2017/000389,filed on Nov. 17, 2017, claiming priority to DE 10 2016 013 994.3 filedon Nov. 23, 2016, each of which is herein incorporated by reference inits entirety.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The invention relates to a method for automatically producing an opticalblend mask which is used in the beam path of a projector in a projectionsystem comprising at least two projectors.

(2) Description of Related Art

Large-area projections, which are produced by so-called projectionsystems, are usually composed of several small projections or partialimages, with the individual projection channels which produce thepartial images being geometrically distorted and blended in a manneradapted to the projection surface. In this way, a correspondinglylarge-scale homogeneous projection or display on the projection surfacecan be produced, for example, by using four projectors, which eachproduce a partial image of the large-area projection.

The information required for the distortion and blending are determined,for example, by a so-called calibration of the overall projection or theprojection system. For this purpose, predefined patterns are projectedonto the projection surface. These patterns mark points on theprojection surface or are designed so that points can be derivedtherefrom. Points derived in this way can be obtained from twointersecting lines or, for example, from one corner of a flat geometricfigure.

The spatial positions (X, Y and Z coordinates) of these marked pointsare measured, for example, with two cameras. Based on this point model,the overlap of the individual projectors can be determined at any pointon the projection screen and a blending for each projector involved inthe production of the large-area projection can be derived therefrom.This blending is realized by the optical blend mask to be producedautomatically.

According to the prior art, the video signals of the individual videochannels of the projectors, which are required for each partial imagegeneration, are varied after image formation by using suitable methodsfor distortion and blending of the video signals. Subsequently, thethus-changed signal is supplied to a projector as an input signal, whichproduces the corresponding partial image on the projection surface. Thisprocess of signal processing or signal change is usually performed foreach of the video channels involved in generating the projection, thatis, for each partial image.

Thus, the changes of the video signals necessary for generating thelarge-area projection on a projection surface, for example, to adapt tothe given projection surface and/or to a blending, are directly appliedto the input signal of each projector and affect the projection of thedisplayed partial images.

Alternatively, it is known that blending can be realized by usingoptical blend masks, which are arranged in the beam path in front of therespective projector.

Regardless of the selected method for the distortion and/or theblending, a recalibration of the projection system is required in theevent that one or more projections or projection channels change, forexample, due to a change in the position of one or more projectors. Thedistortion and the blending must also be corrected with the new data ofthe calibration. In addition to a change in position, a recalibrationand adjustment of the projection system is usually required even whenreplacing a lamp in a projector.

Most projectors are in principle unable to project images or partialimages that are displayed in an image part having no brightness, i.e.which are completely black. This is due to the fact that the lightsource in the projector is always on and that the light path cannot becompletely blocked. As a result, projectors have a so-called residualbrightness, for example when displaying a black screen.

Large-area projections using multiple projectors are usually generatedby at least partially overlapping the partial images in their peripheralareas, thereby producing a so-called blend zone. When such image areasoverlap in the blend zone, the residual brightness of the differentprojectors is unfavorably superimposed. In particular, when representingvery dark image areas in the overlaps of the partial images, theresulting residual brightness is perceived by a viewer as verydisturbing, since the blending has no effect on the residual light.

One solution to the problem of this residual brightness is to reduce orprevent the residual light projection by using an optical blend mask inthe light path. When using, for example, four projectors in a large-areaprojection, four optical blend masks must be provided, i.e. one blendmask for each projector.

In the prior art, these optical blend masks are produced strictlyaccording to a design or according to data from corresponding testprojections. Since the real projection system is always different fromthe design or from an ideally planned projection, the results producedwith these optical blend masks are not always satisfactory.

The reason for the limited functionality of such optical blend masks is,on the one hand, based on the fact that it is very difficult in practiceto construct a projection system exactly according to a given design. Onthe other hand, a so-called individual distortion of a projector causedby inaccuracies in the manufacture of the lenses or lens pairs of aprojector cannot be taken into account at all or only with a substantialdesign effort.

EP 1 422 939 A2 discloses a multi-channel projection system and a methodfor realizing large projection surfaces. To provide a low-cost,large-area projection, a multi-channel projection system with at leasttwo side-by-side or stacked projectors is proposed, with a singleprojector each projecting part of the image. The described projectionsystem is characterized in that an identical input signal representingthe projected total image is applied to the at least two projectors,wherein the pixel resolution of the image to be displayed is greaterthan the resolution of the projection screens of the at least twoprojectors and means are included for electronically moving the image onthe respective panel of the projectors, such that the partial imagesprojected by the individual projectors together produce the total imageto be projected.

WO 2012/076706 A1 discloses an optical blend mask suitable for use inrepresenting an overall picture composed of several overlappingindividual images. The mask includes an arrangement of elongated slidingelements disposed side-by-side on a carrier, with each sliding elementbeing disposed for moving independent of one another along an axis. Itis intended to arrange one or more of these masks, for example, in alight beam of a projector and to thus change the transmission of thelight.

U.S. Pat. No. 6,513,938 B2 relates to a projection system with aplurality of image projection means, in which overlapping partial imagesprojected by the plurality of image projection means are arranged insuch a way that an overall image is produced. The object of thisdocument can be seen to provide a projection system, which makes itpossible to provide an overall picture composed of a plurality ofpartial images with a virtually uniform luminance level over the entireprojection area.

The arrangement includes a plurality of image projection means havingone or more intercepting plates arranged between the image projectionmeans and the projection surface, a luminance detection means, adisplacement calculating means and a control means.

It is disclosed, for example, to use an L-shaped intercepting plate infront of one of four image projection means, which arranged in front ofthe image projection means for displacement in order to adjust therequired luminance. It is also intended to automate this displacement byusing a control signal produced by the control means and correspondingdrive means which are adapted to change the position of the interceptingplate in front of the image projection means in the vertical andhorizontal directions.

EP 1 613 071 A1 disclose a method for providing attenuation of theluminous intensity of an image in an edge region of one of a pluralityof tiled partial images in a total projection displayed by a projectiondisplay system. In particular, the document relates to a method forobtaining improved blending of tiled images displayed by a projectiondisplay system. The method provides both good black-level blending andhigh flexibility.

A first light intensity is changed with an electronic soft-edge deviceover a first fraction of the edge region from a first intensity value toa first fraction of the first intensity value and a gradual change ofthe first light intensity by an optical soft-edge device over a secondfraction of the edge region from a second fraction of the firstintensity value to a third fraction of the first intensity value. Theoptical attenuation of the light intensity of the image should here besubstantially constant in the first fraction and in the second fractionof the edge region. The optical soft-edge device includes at least onemask.

In a particular embodiment, a third fraction is also provided which isarranged between the first and the second fraction and in which both theelectronic soft-edge device and the optical soft-edge device are usedfor influencing the light intensity of the third fraction.

The prior art thus has the disadvantages that no optimal compensationfor the problem associated with residual brightness occurring in theblend zones of a large-area projection is provided. In particular,compensation by using an optical blend mask cannot be adaptedautomatically to the real projection system, which always differs atleast partially from a planned design, and/or to existing distortions ofthe projectors, which likewise always occur in practice. Alternatively,it is only possible to manually adjust the optical blend masks and theblending. For this purpose, elements that influence the propagation ofthe light can be introduced into the beam path, for example in theregion of the blend masks. Such an adjustment of the mask is a manualadjustment operation, which requires an experienced professional and issubject to subjective evaluation.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to provide a method for producing anoptical blend mask, with which an optical blend mask is producedautomatically, which is adapted to an actual individual state of aprojection system as well as to distortions caused by at least one lensof a projector.

The object is achieved by a method having the features according toclaim 1. Further developments are specified in the dependent claims 2 to8.

According to the present invention, it is provided that one or moreoptical blend masks are calculated and produced based on data determinedduring calibration of an actual state of a projection system. Eachoptical blend mask produced in this manner is after its productionarranged or mounted at a defined location in the beam path of anassociated projector.

The uniqueness of the method according to the invention is that blendingis calculated with the aid of projected points located within patternswhose spatial position is measured and which are projected onto theprojection surface and hence also imaged on the projection surfacelocated in the plane of the blend mask, and this calculated blending istransmitted or printed onto an optical blend mask located in the beampath of the projector. For this purpose, the position of a framereceiving the optical blend mask to be produced or of the frame with afront or rear projection surface is measured in the beam path. Torealize this measurement, the frame and/or the front or rear projectionsurface is provided with so-called measuring marks. The position of theblend mask and the blend characteristic on the automatically producedmask can be calculated therefrom.

Optical blend masks produced in this way significantly reduce oreliminate the disadvantage of optical blend masks calculated andfabricated according to a given design and do not require a manualsetup.

This is true both for the inaccuracy of the optical blend mask, whichresults from the fact that it is practically very difficult to constructthe projection system exactly as designed, as well as for the proportionof the inaccuracy caused, for example, by an individual distortion ofeach projector-lens pair.

Either inaccuracies or disturbances are taken into account in a designof the optical blend mask based on data produced by the calibration,i.e. by a measurement of the real actual state of the projection system.In this way, an optical blend mask is produced, which is individuallyadapted to the associated projector and its position with respect to theprojection surface or the projection system.

To produce the data necessary for the production of the optical blendmasks, the following process steps explained below are carried out:

In the first method step, the blending is calculated for each individualprojection based on a measurement of the overall projection. Theblending is calculated in relation to the image signal. In a subsequentmethod step, the distortions of all the projector-lens pairs aremeasured in a so-called blend mask plane, which is located in a plane,typically orthogonal to the optical axis, immediately in front of therespective projector-lens pair. This makes it possible to determine atwhich location and with which distortion the calculated blending must beplaced as an optical blend mask.

For this measurement of the distortion, a front or rear projectionsurface is arranged in front of each projector at a location in the beampath of the projector (blend mask plane) where the optical blend mask tobe produced is used after its production.

Several optical markers are located on this front or rear projectionsurface. The front or rear projection surface is respectively receivedby a holder or frame, which is arranged in front of the respectiveprojector. The front or rear projection surface is removable from theholder and can always be brought exactly in the same position by theholder which is firmly connected to the projector or to another fixedpoint. After completion of the optical blend mask, the optical blendmask is inserted in the holder or frame at the location of the front orrear projection surface.

The markers on the frame form a local reference coordinate system. Thedistortion of the projector is determined in this coordinate system witha calibration. At the same time, the position of the optical blend maskwithin the local reference coordinate system is determined.

To perform the calibration, patterns from which points can be derivedare projected onto the front or rear projection surface arranged in theblend mask plane, which are recorded with a camera and provide anassociation of the front or rear projection surface with the localreference coordinate system.

At least one such camera can be arranged, for example, in front of orbehind the blend mask plane and aligned with the front mask or rearprojection surface arranged in the blend mask plane.

The data of an optical blend mask calculated with the method aretransferred by a suitable technique to a transparent support medium,which may be for example a foil or glass.

In an alternative embodiment, the data obtained in the method may beused, for example, to control a display arranged in front of theprojector at the location of the optical blend mask. For this purpose,any display transmitting the light from the projector can be used. Aswith a manufactured optical blend mask, the data define a brightnessvalue of pixels in the display, with a reduction in the brightness valueresulting in an attenuation of the transmitted light at that location.Such a display can, for example, be a so-called Digital InformationDisplay (DID), a transparent Liquid Crystal Display (LCD), or atransparent Organic Light Emitting Diode Display (OLED).

The produced optical blend masks contain registration marks whichcorrespond exactly to the markers of the frames. The optical blend maskscan thus be fixed in the frame very precisely and thus assume an exactposition in the reference coordinate system.

The optical blend masks are pushed into holders located at a fixedposition in front of the projector. By using this transitive, relativeposition specification, the mask is positioned exactly in the projectionpath, i.e. in the beam path of the respective projector in front of thisassociated projector.

Process Flow:

1. Measuring the projection system and generating the ideal blending

2. Arranging a front or rear projection surface in frames in front of aprojector:

-   -   a) Projecting patterns from which points can be derived,    -   b) Recording the points derived from the pattern, for example by        using a camera,    -   c) Determining the position of the recorded points in the local        reference coordinate system,    -   d) Association with the projected points,    -   e) Calculating the distortion of the projector.

3. Calculation of the optical blending:

-   -   a) Positioning and/or transforming the ideal blending onto the        blend mask layer,    -   b) Filtering or adapting the ideal blending to compensate for        the soft focus in the light path, since the optical blend mask        in the blend mask plane is not in the focal plane of the        projector,    -   c) Optional iterative optimization of the optical blend mask        produced according to steps a) and b), wherein the ideal image        of the blending on the projection surface (blend result) is        simulated and compared with blending expected using the        previously created optical blend mask in order to reduce the        deviations between the ideal image of the blending and the        expected blending,    -   d) Application of the distortion of the projector as determined        in point 2 to the blending.

4. Outputting the generated data for producing the optical blend mask.

In this case, ideal blending is to be understood as blending which wouldallow an absolutely error-free projection of a partial image. The methodautomatically determines data that can be used to produce an opticalblend mask, which very closely approximates the characteristics of theideal blending.

The iterative optimization in point 3c is necessary to reducedisturbances in the representation of the large-area projection 5, whichare caused by diffraction effects and/or distributed light sources.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, features and advantages of embodiments of the inventionwill become apparent from the following description of exemplaryembodiments with reference to the accompanying drawings, which show in:

FIG. 1 a schematic diagram of a large-scale projection onto a curvedprojection surface with several projectors,

FIG. 2 a diagram of the partial images of the large-area projection,

FIG. 3 a diagram of possible signal processing steps for generating ofpartial images of a large-scale projection,

FIG. 4 a large-area projection by using two projectors with a blend zonelocated between the produced partial images,

FIG. 5 a diagram of four optical blend mask masks, which are used ingenerating a large-area projection using four projectors,

FIG. 6 one of several projectors for producing a large area projectionwith an optical blend mask arranged in the beam path of the projector,and

FIG. 7 the positioning of a front or rear projection surface in a framein the beam path of a projector in front of a projection surface.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a projection system 1, which by way of example includesfour projectors 2 a, 2 b, 2 c, 2 d. Each projector 2 a, 2 b, 2 c, 2 dproduces a partial image 4 a, 4 b, 4 c, 4 d on the projection surface 3,which together produce a large-area projection 5. The generation of thislarge-area projection 5 takes place in such a way that a blend zone 9 isformed between two adjacent partial images 4 a, 4 b, 4 c, 4 d, in whichimage portions of the adjacent partial images 4 a, 4 b, 4 c, 4 d aresuperimposed. The image signals or video signals of the partial images 4a, 4 b, 4 c, 4 d required for the control of the projectors 2 a, 2 b, 2c, 2 d are generated in a suitable central signal generating unit, whichwill not be described in detail, since it does not affect the disclosureof the present invention.

The required partial images 4 a, 4 b, 4 c, 4 d are initially generatedin the signal generation unit in such a way that the large-areaprojection 5 can be projected without errors when displayed only on aflat projection surface 6, which is shown in FIG. 1 solely forsimplifying the illustration.

To enable an error-free representation on the curved projection surface3, the entire projection system 1 is calibrated. In this calibration,data is produced which makes it possible to distort the partial images 4a, 4 b, 4 c, 4 d in such a way that they are displayed to a viewer onthe curved projection surface 3 and perceived by the viewer to be trueto the original, without undesired displacements or distortions.

In addition to this distortion of the partial images 4 a, 4 b, 4 c, 4 d,the signals of the partial images 4 a, 4 b, 4 c, 4 d are also changedfor superposition in the blend zones 9 with sufficient accuracy and arehence can no longer perceived by the viewer as individual or separatepartial images 4 a, 4 b, 4 c, 4 d, thus visually displaying a coherentlarge-area projection 5. For this purpose, blending, i.e. an adjustmentof the brightness of the partial images 4 a, 4 b, 4 c, 4 d within theblend zones 9, must be performed.

FIG. 2 shows the partial images 4 a, 4 b, 4 c, 4 d of the large-areaprojection 5 in respective individual images. As can be clearly seen,for example, in the partial images 2 a and 2 b, the partial images 2 aand 2 b can produce a fault-free image only when they partially overlapalong their common side. It can also be clearly seen in the example ofthe partial images 4 a and 4 b that the eye of the illustrated duck mustbe brought into agreement in order to obtain an error-free large-areaprojection 5.

FIG. 3 shows various process flows for distorting and blending thepartial images 4 a, 4 b, 4 c, 4 d. The illustration shows the sectionsImage Generation 15, Distortion 16, Blending 17, Projection 18 and therespective light path 8.

In the first variant (variant 1), after the image or video signals to bedisplayed are generated in an image generation unit 19, for example in acomputer, the partial images 4 a, 4 b, 4 c, 4 d in the section 16 aredistorted for adaptation to the conditions of an uneven projectionsurface 3. This adaptation takes place, for example, in a specialdistortion unit (warp unit) 20 or in a graphics card 21. Subsequently, ablending method is used in section 17 before the adapted signals of thepartial images 4 a, 4 b, 4 c, 4 d in section 18 are each projected onthe projection surface 3 with respective projectors 2 a, 2 b, 2 c, 2 dvia the light paths 8. The blending can be realized in a special blendunit 22 as well as with a graphics card 21.

In the second variant (variant 2), the image or video signals to bedisplayed are already generated by taking into account the distortion aswell as the blending in a single step in a suitable unit, such as acomputer. This unit or this computer represents the image generationunit 19, the distortion unit (warp unit) 20 and the blend unit 22 andapplies the distortion of the signal to be performed in section 16 andthe blending to be performed in section 17. The thus generated signal ofa partial image 4 a, 4 b, 4 c, 4 d is again projected in section 18 ontothe projection surface by using a projector 2 a, 2 b, 2 c, 2 d via thelight path 8.

In the third variant (variant 3), the image or video signals to bedisplayed are generated in an image generation unit 19, such as acomputer or similar unit suitable for data processing. The signalproduced in this way is supplied to a distortion unit (warp unit) 20 ora graphics card where the signal experiences in section 16 acorresponding distortion for adaptation to the projection surface 3.This distorted signal is projected in section 18 by using a projector 2a, 2 b, 2 c, 2 d via the light path 8 onto the projection surface 3,wherein each projector 2 a, 2 b, 2 c, 2 d has an optical aperture mask 7a, 7 b, 7 c, 7 d arranged in the beam path 8 immediately in front of aprojector 2 a, 2 b, 2 c, 2 d, which realizes the blending of the signal.

In the fourth variant (variant 4), the image or video signals to bedisplayed are produced by an image generation unit 19, for example in acomputer, in which the signal also experiences a correspondingdistortion. The computer thus also represents the operation of thedistortion unit (warp unit) 20. In this variant, too, the distortedsignal is subsequently projected in section 18 onto the projectionsurface 3 by using a respective projector 2 a, 2 b, 2 c, 2 d via thelight path 8, with an optical blend mask 7 a, 7 b, 7 c, 7 d, whichimplements blending of the signal, arranged in the beam path 8immediately in front of each projector 2 a, 2 b, 2 c, 2 d.

FIG. 4 shows a large-area projection 5 using two projectors 2 a and 2 b.The partial images 4 a and 4 b produced by the projectors 2 a and 2 bare projected onto the projection surface 3, creating between thepartial images 4 a and 4 b a blend zone 9, in which the partial images 4a and 4 b overlap. As shown in FIG. 4, not only the image contents ofthe partial images 4 a and 4 b overlap in the blend zone 9, but also therespective brightness of the partial images 4 a and 4 b. As alreadydescribed, this overlap of the respective brightness must be compensatedby the blending so that the overlap can no longer be perceived by theviewer as disturbing.

For this purpose, suitable methods are used for blending, i.e. forinfluencing the brightness of the image or video signals of the partialimages 4 a and 4 b in a suitable unit such as a computer or a graphicscard. Alternatively, an optical blend mask 7 a and 7 b can be used inthe beam paths 8 of the respective projectors 2 a and 2 b. Such opticalblend masks 7 a and 7 b are not shown in FIG. 4.

FIG. 5 shows four exemplary optical blend masks 7 a, 7 b, 7 c, 7 d.These four optical blend masks 7 a, 7 b, 7 c, 7 d are intended for aprojection system that produces a large-area projection 5 composed offour partial images 4 a, 4 b, 4 c, 4 d by using four projectors 2 a, 2b, 2 c, 2 d.

The optical blend masks 7 a, 7 b, 7 c, 7 d have areas along two adjacentedges which have dark fields and extend in the shape an “L”. By usingthese dark fields, the brightness of the projected partial images 4 a, 4b, 4 c, 4 d on the projection surface 3 is reduced in the blend zones 9,producing a uniform brightness when the partial images 4 a, 4 b, 4 c, 4d are superimposed. This process of blending is achieved by varying theopacity in the L-shaped fields. The masks 7 a, 7 b, 7 c, 7 d remaintranslucent outside the opaque areas that are not located within theblend zone 9.

FIG. 6 shows part of a projection system 1. The example shows theprojector 2 b of a projection system 1 having four projectors 2 a, 2 b,2 c, 2 d. This projector 2 b projects the partial image 4 b, whichrepresents the upper right-hand portion of the large-area projection 5,onto the curved projection surface 3. In the illustration of FIG. 6, anoptical blend mask 7 b is arranged in the beam path 8 of the projector 2b in a so-called blend mask plane. For affixing the optical blend mask 7b, for example, a frame may be provided, in which the optical blend mask7 b can be arranged. This frame is preferably fixedly connected to theprojector 2 b via a holder. The frame and the holder are not shown inFIG. 6.

FIG. 7 shows positioning of a front or rear projection surface 11 in aframe 10 in the beam path 8 of a projector 2 in front of a projectionsurface 3. The illustration shows a, for example, U-shaped frame 10arranged in front of the projector 2, which is suitable both forreceiving a front or rear projection surface 11 and for receiving anoptical blend mask 7 produced according to the process flow.

The frame 10, which is not limited to the illustrated shape, may forexample have a groove into which the front or rear projection surface 11as well as the mask 7 can be inserted. The frame 10 is firmly connectedto the projector 2 by way of an unillustrated holder or to another fixedpoint and is fixed in its position in this way. Marks 12 which define alocal reference coordinate system are arranged on the frame 10. Theposition of the front or rear projection surface 11 and of the opticalblend mask 7 is defined in this local reference coordinate system.

FIG. 7 shows the frame 10 with a front or rear projection surface 11onto which patterns 13 are projected. Preferably, these patterns 13 canbe points whose relative position with respect to a local referencecoordinate system is determined by using a camera recording the pointsand by an unillustrated computing and control unit.

An imaged projection surface 14, which is generated when the projector 2projects a partial image 4 onto the projection surface 3, is shown onthe front or rear projection surface 11 arranged in the beam path 8 ofthe projector 2. The position and orientation of this imaged projectionsurface 14 located in the blend mask plane is detected in the blend maskplane in accordance with the method and serves to precisely align theoptical blend mask (7) within the image mask plane when the data for theoptical blend mask (7) are generated.

LIST OF REFERENCE NUMBERS

-   1 projection system-   2 a, 2 b, 2 c, 2 d projector-   3 projection surface-   4 a, 4 b, 4 c, 4 d partial images-   5 large-area projection-   6 flat projection surface-   7 a, 7 b, 7 c, 7 d optical blend mask-   8 beam path-   9 blend zone-   10 frame-   11 front or rear projection surface-   12 mark-   13 pattern-   14 imaged projection surface-   15 image generation-   16 distortion-   17 blending-   18 projection-   19 image generation unit (Personal Computer PC)-   20 distortion unit (warp unit)-   21 graphics card-   22 blend unit

The invention claimed is:
 1. A method for automatically generating anoptical blend mask (7), arranged in a beam path (8) of a projector (2)in a projection system (1) comprising at least two projectors (2),comprising the steps of: determining an actual state of the projectionsystem (1) by a calibration, determining an individual distortion of theprojector (2) by using a front or rear projection surface (11) insertedin the beam path (8) between the projector (2) and a projection surface(3) using patterns (13) from which projected points are derived;determining orientation and position of the front or rear projectionsurface (11) within a blend mask plane with respect to a local referencecoordinate system, calculating ideal blending for a partial image (4) tobe projected by the projector (2) in such a way that the ideal blendingis positioned and/or transformed on the blend mask plane, adapting theideal blending to compensate for a soft focus in the beam path (8),applying the determined, individual distortion of the projector (2) tothe ideal blending, and determining data for generating an optical blendmask (7), wherein the optical blend mask (7) generated according to thedetermined data is used at a location in the beam path (8) of theprojector (2) at which the front or rear projection surface (11) waspositioned when determining the individual distortion of the projector(2).
 2. The method according to claim 1, wherein the determining theindividual distortion of the projector (2) includes the following methodsteps: projecting the patterns (13), from which the projected points arederived, recording the projected points derived from the pattern (13) byusing a camera, with position and alignment and/or orientation of thecamera in the local reference coordinate system being known, determiningthe projected points derived in the local reference coordinate system,recording of the determined projected points derived in the localreference coordinate system, calculating the distortion of the projector(2) based on the recorded projected points derived.
 3. The methodaccording to claim 1, wherein the calculation of the ideal blending alsocomprises the method steps of iteratively optimizing the data of thegenerated optical blend mask (7), wherein ideal imaging of the blendingon the projection surface (3) is simulated and compared with an expectedblending using the data of a previously generated optical blend mask (7)so as to reduce discrepancies between the ideal image of the blendingand the expected blending.
 4. The method according to claim 1, whereinthe data for generating the optical blend mask (7) includes informationabout a number of pixels arranged horizontally and vertically in theoptical blend mask (7) and their intensity on the optical blend mask(7), with which a brightness reduction is performed.
 5. The methodaccording to claim 1, wherein the data for generating the optical blendmask (7) is determined for two or more optical blend masks (7).
 6. Themethod according to claim 1, wherein the data of the generated opticalblend mask (7) is used to control a printing method for producing aprinted optical blend mask (7).
 7. The method according to claim 1,further comprising the steps of: transmitting an image to be displayedvia the projector (2); and controlling the transmitted image to bedisplayed via the generated optical blend mask (7).
 8. The methodaccording to claim 1, wherein the projection system (1) includes a framepositioned in the beam path; the generated optical blend mask (7) or thefront or rear projection surface (11) each being receivable in theframe; markings on the frame forming the local reference coordinatesystem.